Moon Phases: Crash Course Astronomy #4

Besides the Sun, the Moon is the most obvious object in the sky. Bright, silvery, with tantalizing

features on its face, it’s been the target of imagination, poetry, science,

and even the occasional rocket.

If you pay even the most cursory attention to it, you’ll see that it changes every

day; sometimes it’s up in the day, sometimes at night, and its shape is always changing.

What causes this behavior?

The Moon is basically a giant ball of rock 3500 kilometers across hanging in space. Its

surface is actually pretty dark, with about the same reflectivity as a chalkboard or asphalt.

However, it looks bright to us because it’s sitting in full sunlight; the Sun illuminates

it, and it reflects that light down to us here on Earth.

And because it’s a sphere, and orbiting the Earth, the way we see it lit by the Sun

changes with time. That’s what causes its phases: geometry.

The important thing to remember through all this is, because the Moon is a ball and in

space, half of it is always illuminated by the Sun!

This is true for the Earth, too, and every spherical object in space; half faces the

Sun, half faces away. We call the part facing the Sun the daylight or bright side, and the

half facing away the night or dark side.

The phase of the Moon refers to what shape the Moon appears to us; how much of it we

see illuminated from the Earth. The key to all this is this line, dividing the lit day

side from the unlit night side. We call that line the terminator.

If you’re facing the moon, with the sun behind you, you’re seeing the half of the

moon that is fully illuminated by sunlight and it looks full. If you’re off to the

side you see half of the lit side and half of the dark side and we say the moon is half

full. If the sun is on the other side of the moon, you’re look at the unlit half, and

it looks dark. Now, mind you, I haven’t moved anything except our point of view here,

so at all times the Moon is always half lit, and half dark. Remember that.

The phase of the Moon we see depends on from what direction the sunlight’s hitting it,

and the angle we see that from Earth.